One example of an X-ray diagnostic device is an X-ray CT (Computed Tomography) scanner. This CT scanner is composed of an X-ray tube and an X-ray detector. The tube is designed to emit a fan-shaped beam of X-rays, i.e., an X-ray fan beam. The detector is composed of many X-ray detecting elements arrayed side-by-side. The scanner operates as follows: The X-ray tube emits a series of X-ray fan beams one after another onto the X-ray detector. Each time a fan beam is incident on the surface a tomographic layer, it is oriented at a predetermined angle, for example, 1 degree, from the previous one. The scanner collects thereby data on absorptions of X-rays. Subsequently, the data is analyzed by a computer. An absorptance at an individual location on the surface of a tomographic section is thereby calculated. An image is thereby formed according to the absorptances.
Conventionally, as this X-ray detector, a xenon gas detector has been used. This detector operates as follows: The detector has a gas chamber filled with a xenon gas. This detector has many electrodes arrayed therein. First, a voltage is applied between each pair of electrodes. Subsequently, the detector is irradiated by X-rays. The xenon gas is thereby ionized. A current signal is thereby output. The level of the signal corresponds to the intensity of the emission of X-rays. This enables an image to be formed. However, the interior pressure of the xenon gas filled chamber is high. Therefore, the window of the chamber must be thick. This decreases use efficiency of X-rays. In addition, in order to obtain a high resolution CT scanner, the thickness of an electrode plate must be very small. However, if the electrode plate is thin, vibration from the exterior vibrates the electrode plate. This generates noise. This constitutes a problem.
On the other hand, the following type of X-ray detector has been developed and has been put to practical use. The detector is composed of a specific type of scintillator and of a silicon photodiode. The scintillator is made of a fluorescent material such as a CdWO4 single crystal, a (Y, Gd)2O3:Eu, Pr ceramic, a Gd2O2S:Pr, Ce, F ceramic (referred to as “GOS:Pr” hereinafter), or a polycrystalline ceramic made of an oxide (referred to as “GGAG:Ce” hereinafter) having a garnet structure primarily containing gadolinium oxide, gallium oxide, and aluminum oxide, and cerium oxide. In this X-ray detector, when absorbing X-rays, the scintillator emits fluorescent light. Subsequently, the silicon photodiode detects this light. Thus, X-rays are detected. The fluorescent material constituting the scintillator emits light having a wavelength corresponding to an energy level produced by an activator element added to the matrix thereof. If this wavelength is not smaller than 500 nm and corresponds to a visible light, this causes the silicon photodiode to acquire high detection efficiency. This in turn causes the X-ray detector to acquire particularly high sensitivity.
A compositional formula expressing a fluorescent material contains the colon “:”. A matrix is indicated on the left side thereof. An activator ion is indicated on the right side thereof. If an X-ray detector is formed by using such materials, this makes it possible to miniaturize an X-ray detecting element, to increase the number of channels and to obtain a high resolution image. In general, such a fluorescent material is required to be a highly homogenous material, to vary little from one piece thereof to another in the X-ray characteristics, to be little deteriorated by radiation, to keep the fluorescence characteristics substantially unchanged when environmental conditions such as temperature are changed, to be easy to machine, to be hardly deteriorated by machining, to absorb substantially no moisture, to be not deliquescent, to be chemically stable, etc.
In this type of X-ray detector, the more a scintillator absorbs X-rays, the higher the light intensity (referred to also as the “fluorescence intensity” hereinafter). The higher the light intensity becomes, the higher the sensitivity of the detector. Thus, the high intensity of fluorescence requires absorbing X-rays sufficiently. Given a constant amount of X-ray radiation, the less a scintillator absorbs X-rays, the more the scintillator transmits X-rays. This causes noise in a silicon photodiode. This in turn decreases the sensitivity. The amount of transmission of X-rays through a scintillator may be decreased by increasing the thickness of the scintillator. However, an increase in the thickness thereof hinders miniaturization of an X-ray detecting element. This increases the cost. Therefore, it is desirable that a fluorescent material be thin and have a large X-ray absorption coefficient. In addition, the lower the light transmittance in a fluorescent material, the less likely the emitted light reaches the silicon photodiode. This substantially decreases the fluorescence intensity. Therefore, in order to obtain high fluorescence intensity, a fluorescent material to be used as a scintillator is required (1) to have a large X-ray absorptance and (2) to have a high light transmittance for fluorescent light.
First, high resolution is required in X-ray computed tomography. High resolution may be achieved by miniaturizing an X-ray detecting element. Second, X-ray CT must avoid effects due to movements of the subject. This may be achieved by shortening scan time. All this causes an X-ray detecting element to have a shortened integration time. This in turn causes the total amount of absorbed X-rays within an integration time to be decreased. This makes high fluorescence efficiency (high fluorescence intensity) necessary. The time resolution of an X-ray detecting element may be increased in that the intensity of fluorescence (afterglow) after the termination of X-ray radiation decreases instantaneously and considerably. This requires a small time constant for the decay of fluorescence and a low level of afterglow. The fluorescence decay time constant refers to a time period in which the intensity of fluorescence after the termination of X-ray radiation has decayed to 1/e of the intensity of fluorescence during X-ray radiation. The level of afterglow is the ratio of the fluorescent intensity a predetermined time after the termination of X-ray radiation to the fluorescent intensity during X-ray radiation. If the decay occurred perfectly exponentially, a decrease in the decay time constant would necessarily cause a decrease in the level of afterglow. However, actually, the level of afterglow does not decrease exponentially. Therefore, a high-performance X-ray CT scanner with the low level of afterglow requires a fluorescent material that has a small decay time constant and the low level of afterglow. Table 1 shows the fluorescent intensity, the decay time constant, and the level of afterglow 30 ms after the termination of X-ray radiation for various types of fluorescent materials that have been conventionally used.
TABLE 1Decay TimeFluorescenceConstantAfterglowCompositionMaterialDensityIntensity(μs)(% at 30 ms)CdWO4Singlecrystal7.99565.00.002Gd2O2S:Pr, Ce, FPolycrystal7.281003.00.01(Y, Gd)2O3:Eu, PrPolycrystal5.9210010000.01Gd3Ga5O12:Cr, CePolycrystal7.09721400.01Gd3Al3Ga2O12:CePolycrystal6.4695appr. 0.10.01Footnote 1: The fluorescence intensity, the decay time constant, the level of afterglow were measured using a silicon photodiode (S2281 manufactured by Hamamatsu Photonics)Footnote 2: The fluorescence intensity is an relative value with reference to the fluorescence intensity of Gd2O2S:Pr, Ce, F.
Among the above materials, Gd3Al3Ga2O12:Ce (GGAG:Ce) emits fluorescent light. This occurs in that Ce acts as an activator Ce. That is, this is caused by the allowed transition of Ce3+ from 5d level to 4f level. Therefore, Patent Documents 1 to 7 disclose a polycrystalline material made of GGAG:Ce as a fluorescent material.
Among the above patent documents, Patent Documents 6 and 7 specify, in particular, the content of Si in GGAG:Ce. Patent Document 6 discloses a GGAG:Ce scintillator containing 0.001 to 5.0 (excluding 0.001 and 5.0) mol of the group IVb elements including Si, in terms of molar ratio. Patent Document 7 discloses a GGAG:Ce scintillator containing smaller than 100 ppm of Si by weight. The decreased content of Si is due to the increased light transmittance and the decreased level of afterglow.